A time constant is a parameter used to characterize the response of a system to a change in an input condition. The value of a time constant used to characterize electronic systems is calculated by multiplying a value of resistance R in the system times a value of capacitance C in the system and is referred to as an RC time constant. Methods for calculating and measuring responses by electronic systems to different input signals and RC time constants are well known.
For some applications, a preferred response by an electronic system corresponds to a large value for an RC time constant. For example, a control circuit having a large RC time constant will have a response related to low-frequency components of an input signal while suppressing responses to high-frequency components of the input signal. A response related to a large RC time constant may be advantageous, for example, in a control circuit for maintaining a direct current (DC) voltage within selected limits in the presence of high-frequency noise.
In a system having space for discrete electronic components, an RC time constant having a large value may be implemented by selecting resistors and capacitors with sufficiently large values to form the desired mathematical product. For discrete resistors in particular, the package size and cost of a resistor with a large resistance value is about the same as the package size and cost of a resistor with a small resistance value. However, in an integrated circuit, implementing a large value for an RC time constant may not be so easily accomplished. The amount of die area required for forming a resistor or a capacitor in an integrated circuit increases as the value of resistance or capacitance increases. As a result, implementing large time constant values with structures formed in an integrated circuit by methods known in the art may consume so much die area as to be impractical or uneconomical. Although techniques for increasing an effective amount of resistance or capacitance in an integrated circuit are known in the art, the resulting values of resistance and capacitance may still be too small to achieve a desired large time constant value in an integrated circuit. Furthermore, techniques known in the art for implementing a large time constant by increasing an effective amount of resistance may result in substantial variation in time constant values because of variations in manufacturing process variables and variations in an integrated circuit's operating temperature.
Implementing a large RC time constant in an integrated circuit manufactured by bipolar semiconductor processes illustrates some of the difficulties encountered with solutions known in the art. For example, a time constant of 0.1 millisecond (ms), a large RC time constant by conventional practices for integrated circuit design but small compared to time constants related to many physical processes, could be implemented with a 1 megohm (MΩ) resistor and a 100 picoFarad (pF) capacitor. Forming a 1 MΩ resistor as a metal sheet resistor in a typical bipolar process having a density of 50Ω per square and a minimum line width of 2 micrometers (μm) would require a serpentine line 2 μm wide by 40,000 μm, that is, 40 millimeters long. Such a large structure would be very expensive in terms die area and integrated circuit package size. A 100 pF capacitor implemented in a metal-insulator-metal (MIM) structure such as in a pure bipolar process would also require a large area on the integrated circuit's die. As a result, integrated circuits which include circuit functions requiring large time constants often find it necessary to use one or more discrete capacitors or resistors located externally to and electrically connected to the integrated circuit to set the value of the time constant.
Combining discrete external components with internal components in an integrated circuit to implement a time constant with a large value has several disadvantages. Manufacturing and inventory costs are higher for a combination of an integrated circuit and discrete components than for an integrated circuit alone. Reliability of a system with a combination of an integrated circuit and discrete external resistors and capacitors may be lower than reliability of an integrated circuit without such external components. The number of connection pins on the integrated circuit package may need to be increased to permit electrical connections to external resistors or capacitors, and a larger package may be needed to provide space for the higher pin count. Also, a system having a combination of an integrated circuit and discrete components may be more susceptible to electrical noise and interference than a system operating without such discrete components.
What is needed is a system for implementing large time constants only with components included in an integrated circuit, that is, without external resistors or capacitors. What is also needed is a system for implementing large time constants while reducing sensitivity to changes in process variables related to manufacture of an integrated circuit and changes in an integrated circuit's operating temperature. What is further needed is a system for implementing large time constants that does not require a large amount of die area in an integrated circuit.